docs: def get_fee(speed=FEE_SPEED_MEDIUM):
      """Gets the recommended satoshi per byte fee.
  
      :param speed: One of: 'fast', 'medium', 'slow'.
      :type speed: ``string``
      :rtype: ``int``
      """
      if speed == FEE_SPEED_FAST:
          return DEFAULT_FEE_FAST
      elif speed == FEE_SPEED_MEDIUM:
          return DEFAULT_FEE_MEDIUM
      elif speed == FEE_SPEED_SLOW:
          return DEFAULT_FEE_SLOW
      else:
          raise ValueError('Invalid speed argument.')